1. Field of the Invention
The present invention relates generally to implantable cardiac devices and, more particularly, to implantable cardiac devices capable of hemodynamic measurement.
2. Background Art
An implantable cardiac device is a medical device that is implanted in a patient to monitor electrical activity of the heart and to deliver appropriate electrical and/or drug therapy, as required. Implantable cardiac devices include, for example, pacemakers, cardioverters and defibrillators. The term “implantable cardioverter defibrillator” or simply “ICD” is used herein to refer to any implantable cardiac device capable of delivering therapy to prevent or terminate a fast heart rate or a tachycardia. An ICD employs a battery to power its internal circuitry and to generate electrical therapy. The electrical therapy can include, for example, pacing pulses, cardioverting pulses and/or defibrillator pulses. This is in contrast to a “pacemaker” which is an implantable device specifically intended to treat slow heart rates or bradycardia. However, an ICD provides all the features of a pacemaker. An ICD also includes electrical sensing in its circuitry that monitors the electrical activity of the heart. While performing hemodynamic measurements would be advantageous, at present commercially available ICDs do not perform this function.
Hemodynamic status describes whether the heart is pumping blood sufficiently to ensure adequate perfusion of vital organs. Delivering anti-arrhythmia therapy, via an ICD for example, according to hemodynamic status of an arrhythmia provides several important benefits. Hemodynamically unstable arrhythmias are treated quickly and aggressively, which improves the chance of successful arrhythmia termination. Hemodynamically stable rhythms, during which the patient is most likely to be conscious, are treated with lower voltage therapies. This approach minimizes the risk of painful shocks and conserves battery power of an ICD while at the same time increases the probability of successful arrhythmia termination. Treating a patient according to hemodynamic status measurement may become even more important as the clinical indications for ICD implant become broader.
Hemodynamic status can be measured, for example, by one or more physiologic sensors located within an ICD. One example of a physiologic sensor used for hemodynamic measurement is a hemodynamic sensor. An example of a hemodynamic sensor is an acoustic sensor, which uses an acoustic transducer responsive to heart sounds to detect the hemodynamic status of a patient. For a more detailed description of hemodynamic measurement, including the use of acoustic sensors, see U.S. Pat. No. 6,477,406 B1 (Turcott), which is incorporated herein by reference. Another example of a hemodynamic sensor is a photoplethysmography sensor, such as that described in U.S. Pat. No. 6,409,675 (Turcott), which is incorporated herein by reference. Other types of hemodynamic sensors include intravenous or intracavitory pressure and flow sensors, or optical or mechanical plethysmography sensors. Right ventricular (RV) pressure, for example, is described further in U.S. Pat. Nos. 3,614,954 (Mirowski et al.); 3,942,536 (Mirowski et al.); 4,774,950 (Cohen); 4,967,749 (Cohen); 5,899,927 (Ecker et al.); 6,208,900 (Ecker et al.); 6,221,024 (Miesel); and 6,264,611 (Ishikawa et al.), which are incorporated herein by reference.
Conventional approaches to hemodynamic sensing face a common problem in that it is difficult to provide accurate and reliable data in the face of mechanical artifact. For example, hemodynamic status is particularly difficult to measure due to motion-induced artifact associated with a change of posture and chest compressions that are likely to accompany a significant arrhythmia. Other sources of noise that may affect hemodynamic measurement, depending on the type of hemodynamic sensor used, include electrical noise, external light, changes in atmospheric pressure, and radiated energy.
What is needed is a device, such as an ICD, that is immune from motion and noise during hemodynamic measurement and can reliably use such hemodynamic measurement to deliver an appropriate electrical therapy.